Consecutive crowbar circuit breaker

ABSTRACT

A circuit breaker is inserted in a high-current, high-voltage DC power line between the source and the load. The circuit breaker comprises a parallel connection of a transfer switch, an electronic switch, a first consecutive interrupter having a preferably nonlinear resistor in series therewith, and a second consecutive interrupter having a preferably nonlinear resistor in series therewith. Furthermore, a surge capacitor and its suppression resistance are serially connected in parallel around the second consecutive interrupter. When a fault occurs, the transfer switch is opened and is deionized during conduction of the electronic switch. Offswitching of the electronic switch causes current flow through the two parallel consecutive interrupters with their series resistances to decrease circuit current. During the period that the consecutive interrupters are sequentially opened, the electronic switch is conductive so that the consecutive interrupters can be deionized.

United States Patent [151 3,641,358 Lian et a1. 1 1 Feb. 8, 1972 [54]CONSECUTIVE CROWBAR CIRCUIT 3,476,978 11/1969 Greenwood ..307/ 176 XBREAKER 3,515,940 6/1970 Hobson ..307/ 136 X 3,522,472 8 1970 B tholtz..3l7 11 C X 72 Inventors: Kenneth T. Lian; Willis F. Long, both of I mI Thousand oaks Cahf' Primary Examiner--Robert K. Schaefer [73]Assignee: Hughes Aircraft Company, Culver City, AssistantExaminer-William J. Smith Calif. Attomey-James K. Haskell and Allen A.Dicke, Jr.

[22] Filed: June 10, 1970 [57] ABSTRACT [21] APPl' A circuit breaker isinserted in a high-current, high-voltage DC power line between thesource and the load. The circuit [52] US. Cl. ..307/ 136, 317/11 Cbreaker comprises a parallel connection of a transfer switch, [51] Int.Cl.. ..H0lh 9/30 an electronic switch, a first consecutive interrupterhaving a 1 1 Fwd Search 1 1 11 11 preferably nonlinear resistor inseries therewith, and a second 307/136 consecutive interrupter having apreferably nonlinear resistor in series therewith. Furthermore, a surgecapacitor and its sup- [56] Ream cued pression resistance are seriallyconnected in parallel around the second consecutive interrupter. When afault occurs, the UNITED STATES PATENTS transfer switch is opened and isdeionized during conduction 3,534,226 10/ 1970 Lien ..317/1 1 C of theelectronic switch. Offswitching of the electronic switch 3,475,620 10/1969 Murray et a1. ..307/136 causes current flow through the twoparallel consecutive inter- 02 1/ 1964 Cable 317/11 X rupters with theirseries resistances to decrease circuit cur 3,237,030 2/1966 Cobum307/136 X rent. During the period that the consecutive interrupters are3,249,810 5/1966 Strom et .....317/l1 A sequentially opened, theelectronic switch is conductive so 3,401,303 9/1968 walker 1 that theconsecutive interrupters can be deionized. 3,430,063 2/1969 Webb 307/1363,448,287 6/1969 Giammona ..307/ l 36 12 Claims, 4 Drawing FiguresCONSECUTIVE CROWBAR CIRCUIT BREAKER BACKGROUND OF THE INVENTION Thisinvention is directed to a consecutive crowbar circuit breaker of suchnature as to permit the stopping of current flow in high-current,high-voltage DC powerlines.

The multiphase transmission of electric power at a frequencycorresponding to the generating source and the load equipment is widelyused at present. The employment of alternating current is desirablebecause it permits the use of transformers to change voltages from avalue suitable for generation, to a value suitable for transmission, toa value suitable for distribution, and finally a value suitable for use.

Increasing power demands by the technologically advancing community hasresulted in a transmission at higher voltages and for longer distances.The transmission line reactance is such that further increase oftransmission line length or voltage becomes uneconomic. The user mustpay for even greater power losses as distances and voltages areincreased.

As a result of this, efforts have been made to transmit electric powerby direct current links. Direct current is much more satisfactory from areactance viewpoint for subsea or subterranean installations. Thus,modern interisland ties have been DC in nature. The same considerationsapply to longer overground installations, and to undergroundinstallations. With increasing size of urban centers, and with theaesthetic demands that lines be placed underground wherever possible, itis expected that future urban transmission lines will be subterranean.This requirement points to the need for employing direct currenttransmission.

Of course, the resistant loss of a DC line is decreased by increasingthe voltage and decreasing the line current. However, switching andinterrupting devices for such higher voltage, and especiallyhigh-current DC transmission lines have previously been unavailable.

SUMMARY In order to aid in the understanding of this invention it can bestated in essentially summary form that it is directed to a consecutivecrowbar circuit breaker for the breaking of a highwoltage, high-currentDC circuit. The circuit breaker comprises a transfer switch in the DCline which normally carries the DC current. Parallel to the transferswitch are an electronic switch, a first consecutive interrupter havinga resistor in series therewith and a second consecutive interrupterhaving a resistance in series therewith. The transfer switch, theelectronic switch and the consecutive interrupters are controlled tooperate in proper sequence upon fault detection.

Accordingly, it is an object of this invention to provide a consecutivecrowbar circuit breaker which is capable of breaking a high-voltage,high-current DC circuit. It is a further object to provide a circuitbreaker which will permit the employment of DC transmission lines withcircuit breakers therein to control circuit faults. It is another objectto provide a circuit breaker having a single electronic switch therein,which electronic switch is closed during the opening steps of contactsin parallel thereto so that arcing is limited by the voltage drop on theelectronic switch. It is still another object to provide a consecutivecrowbar circuit breaker which consecutively opens contacts, with thecontacts opening after the first one having resistance in seriestherewith for the absorption of circuit energy. It is a further objectto employ nonlinear resistances in series with consecutively openinginterrupters so that a maximum amount of circuit energy can be absorbedper interrupter to limit the number of consecutive interrupters to two.It is another object to provide a circuit which employs an electronicswitch which can be opened or interrupted to interrupt current flowingtherethrough so that arcing from opening mechanical contacts isminimized.

Other objects and advantages of this invention will become apparent fromthe study of the following parts of the specification, the claims andthe attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic drawing of ahigh-voltage, high-current DC circuit having the consecutive crowbarcircuit breaker of this invention connected therein.

FIG. 2 is a graph of circuit current vs time during the opening sequenceof the circuit breaker of this invention.

FIG. 3 is a graph showing the voltage across the DC buses, during thesequence shown in FIG. 2.

FIG. 4 is a diagrammatic showing of the control equipment which controlsthe switches and interrupters.

DESCRIPTION FIG. 1 illustrates a high-voltage DC circuit with theconsecutive crowbar circuit breaker of this invention incorporatedtherein. The circuit is generally indicated at 10 and the circuitbreaker is generally indicated at 12.

The circuit 10 comprises positive bus 14 and return bus 16. Connectedtherebetween is a high-voltage, high-current DC power source 18 which isconveniently illustrated as being a battery. However, as is well knownto those in the art, the power source usually comprises an engine orturbine driven multiphase AC generator which supplies power totransformers. The transformers increase the voltage and supply therectifiers which are connected between positive bus 14 and return bus16. The preferred example given in this specification is for a 400megawatt system, because that power level appears to be appropriate forfuture use in power generation for adjacent urban environments. It is alevel which might be used in underground transmission of power fromnearby generating plants to urban areas. In such an example, the normalcurrent is 1,000 amperes, as illustrated by the ordinate in FIG. 2 whereeach of the numerals indicates 1,000 amps. Furthermore, the normalvoltage level between the positive bus 14 andreturn bus 16 is 200kilovolts, as illustrated by the ordinate in FIG. 3 where the numbersillustrate thousand volts. Furthermore, return bus 16 is preferably atground potential and a duplicate circuit 10 is provided with a negativebus at 200 kv.and a duplicate of the circuit breaker 12. In other words,FIG. 1 illustrates half of a system which, for purposes of example andillustration throughout this specifica tion, is a 400 mw. system.

Inductance 20 is serially connected with power source 18. Inductance 20represents the inductance of the entire circuit. The circuit inductancelimits the change in current with respect to time, and should the normalcircuit inductance be too low, an additional inductor can be installedfor smoothing and for limiting the rate of current increase in faultconditions. In the specific example of this specification, the circuitinductance is one-half henry so that at the 200 kv. power source voltagethe rate of change of current with respect to time upon occurrence of afault is 400 amps per millisecond.

Serially connected in the line is fault sensor 22, transfer switchcontacts 24 and load 26. Load 26 can be any conventional commercial loador any special load which employs the power produced by the powersource. Thus, load 26 can include inverters, transformers anddistribution equipment to the ultimate load. Lines 28 and 30 representtransmission portions of the positive bus 14 and return bus 16,respectively, which transmit the power from the source to the load.Thus, the circuit breaker 12 is preferably adjacent the source 18 andtransmission over a distance occurs in lines 28 and 30.

Connection 32, with its switch 34, between lines 28and 30, represents ashort circuit such as might occur at the input to load 26 or in thelines 28 and 30 leading thereto. Closure of the switch 34 represents aninadvertent short circuit and thus, connection 32 with its switch isschematically illustrative of other types of highly conductiveelectrical connections between lines 28 and 30.

Buses 36 and 38 are part of the circuit breaker l2 and are connected topositive bus 14 on opposite sides of transfer switch contacts 24.Connections between these buses 36 and 38 thus present parallelconnections across transfer switch contacts 24. Electronic switch 40 isconnected between these buses. The series combination of firstconsecutive interrupter 42' and its series resistance 44 are connectedtherebetween. Similarly, second consecutive interrupter 46 and itsseries resistor 48 are connected therebetween. If required, additionalseries combinations of consecutive interrupters and resistances can beconnected therebetween for successive operations. Finally, suppressionresistance 50 and capacitor 52 are serially connected together and areconnected in parallel around second consecutive interrupter contacts 46.

Referring to FIG. 4, fault sensor 22 senses a fault condition and isconnected to control unit 54 which contains control circuitry tofunction as is hereinafter described. The output of control unit 54 goesto transfer switch contact operator 56, electronic switch operator 58,first consecutive interrupter contact operator 60, and secondconsecutive interrupter contact operator 62. I

Fault sensor 22 is any convenient and conventional fault sensor which isresponsive to voltage between buses 14 and 16, is responsive to rate ofchange of the voltage between the buses, is responsive to current in bus14 or is responsive to the change in current with respect to time in bus14, or a combination of these signals. Suitable fault sensors are shownin the following U. S. Pat. Nos: 3,353,17l; 3,419,791; 3,463,998;3,471,784; 3,473,106; 3,475,653; 3,478,352; and 3,489,920. Any one ormore of these can be employed as fault sensor 22. The particular faultsensor is not critical to the invention and any conventional faultsensing means can be employed.

Transfer switch contact operator 56 and its contacts 24 are in thenature of those found in a conventional circuit breaker such as shown inU. S. Pat. No. 3,268,687 operating in SF to generate sufficient arcvoltage. The requirements are that the transfer switch contacts 24 beable to carry 1,000 amps when closed (the maximum current in theexemplified DC circuit of this specification), and to withstand withoutconduction the surge voltage of the circuit. For the purpose of theexample, the surge voltage is selected to be 1.7 times the normalcircuit voltage. With the normal circuit voltage at 200 kv., the surgevoltage is 340 kv. in accordance with this example. Thus, when openedand deionized, the transfer switch contacts 24 must be able to withstandan applied DC voltage of 340 kv.

The electronic switch 40 can be either a crossed field switching deviceor a liquid metal cathode-switching device, both of which are describedin detail in patent application Ser. No. 681,632, filed Nov. 9, 1967,now U. S. Pat. No. 3,5 34,226, granted Oct. 13, I970. The requirement ofthe electronic switch 40 is that it be able to turn on with voltageapplied thereacross from a fairly small voltage value to a voltage valueabove the normal rated circuit voltage. In the present instance, onswitching should be able to be accomplished for any value of voltageapplied thereacross from its own minimum voltage drop to at least 220kv., as seen in FIG. 3.

With respect to conduction, it must be able to conduct up to four timesthe normal circuit current. In accordance with the example of thespecification, the maximum current has been chosen to be limited to fourtimes normal current, which is consistent with surge voltages of 1.7times normal voltage, and a iz-henry system inductance. Thus, electronicswitch 40 must be capable of conducting up to 4,000 amps.

Furthermore, the electronic switch 40 must be capableof offswitchingagainst this current. In order to be satisfactory for operation in thecircuit of this example, the increase in voltage withstood by the switch40 with respect to time should be at least 1.0 kv. per microsecond. Thecrossed field switch and the liquid metal cathode switch of theabove-identified patent are satisfactory for this purpose. Of course,the electronic switch 40 may represent one or more serially connectedelectronic switches as described in the patent, to provide the desiredstandoff voltage for offswitching capability should the characteristicsof electronic switch devices of commercial configuration so indicate.

Electronic switch operator 58 is connected to the electronic switch 40in order to control its on and off switching. As is described in U. S.Pat. No. 3,534,226 the electronic switching devices can be controlledfor on and off switching.

First consecutive interrupter contacts 42 and its operator 60 are. againin the nature of a circuit breaker such as is shown in U. S. Pat. No.3,268,687, but for this use, a circuit breaker having that fast anoperating speed is not necessary. The contacts must be capable ofwithstanding 340 kv. when open and must be capable of conducting notmore than 3,200 amps when closed. The second consecutive interrupterwith contact 46 and operator 62 is identical to the first consecutiveinterrupter. These interrupters share the 4,000 amp maximum currentanddivide them in accordance with their series resistors 44 and 48.

Series resistors 44 and 48 are shown as being nonlinear re-- sistors.Such are preferable, for with nonlinear resistors the circuit breaker 12of this invention is able to accomplish the circuit breaking function ofthe example with only the first and second consecutive interrupters 42and 46. If linear resistors were employed instead of nonlinear resistors44 and 48, at least three consecutive interrupters would be required.Series resistors 44 and 48 are silicon carbide devices. These resistorsdivide the maximum current between contacts 42 and 46 on the basis of2,700 and 1,300 amps.

Surge capacitor 52 is of conventional oil filled character and has avalue of about 2.0 microfarads in the example of the specification. Itis capable of withstanding the 340 kv. voltage to arrest the finalvoltage surge. Its surge suppression resistor 50 has a value of ohms andis capable of carrying 700 amps in surge suppression duty.

In normal operation of circuit 10, power source 18 is supplying l,000amps of current through inductance 20, fault sensor 22, closed contacts24 of the transfer switch and through load 26. The voltage drop acrossthe load is the nominal circuit value of 200 kv. Under thesecircumstances, the contacts 42 and 46 of the first and secondconsecutive interrupters are closed. Additionally, the electronic switch40 is in standby condition so that when a voltage is appliedthereacross, it will be conductive. This is the state of affairsillustrated in FIGS. 2 and 3 along the abscissa from the intersection totime point a.

At this point in time, a fault appears short-circuiting lines 28 and 30,as represented by the closing of switch 34. This fault causes a drop involtage to near zero, and an increase in current as limited by the valueof inductance 20. Current increases at the rate of 400 amps per ms., aspreviously described. Sensor 22 senses the increase or the rate ofincrease of current, or the decrease or the rate of decrease of voltagebetween buses, or the combination of these signals to determine that afault has occurred. Such determination occurs at point b along theabscissa of the graphs of FIGS. 2 and 3. This sensing by sensor 22causes control unit 54 to signal operator 56 to open transfer switchcontacts 24. These switch contacts open, and as they open an arc isdrawn and voltage drop occurs thereacross. The electronic switch 40 isin its standby condition wherein it is ready to conduct as soon as thereis sufficient voltage thereacross. The opening of contacts 24 and thevoltage drop in the are thereacross provides this sufficient conductionvoltage for electronic switch 40. In the case of a single crossed fieldtube conducting current of the magnitude indicated, an appropriatevoltage drop is 500 v. The voltage drop across the electronic switch isindicated along the abscissa in FIG. 3 between b and c. The voltage dropof a liquid metal are switching device would be somewhat lower.

Since the current is increasing from a to c, this interval must be keptto an absolute minimum. Thus, a fast reacting sensor operating contactshaving a minimum opening time is employed. The electronic switch 40remains conducting a sufficient length of time from b to c to permit thetransfer switch contacts 24 to fully open and deionize so that they canwithstand the peak 340 kv. which will be applied thereto. For thetransfer switch 24 above, the time interval from b to c is 1-5 ms.

At the time point 0, control unit 54 operates electronic switch operator58 to turn off the electric switch 40. This causes the voltage surge atpoint c since all current must pass through the paralleled first andsecond consecutive interrupter contacts 42 and 46, and their respectiveresistances. The value of these resistances is such as to cause areduction in current from time point c to point d.

During this time period, the voltage drop does not decay in proportionto current because of the nonlinear characteristics of resistances 44and 48. Ideally, the maximum energy absorption would be obtained in theshortest time if the top of the voltage curve was level. However, thecurve shape illustrated is the most satisfactory shape available withpresent devices. When the voltage and current have reduced to such anamount that first consecutive interrupter contacts 42 can be openedwithout surging the voltage above the permissible 340 kv., control unit54 causes the first consecutive interrupter contacts 42 to be opened andagain causes conduction of electronic switch 40 for a short time period.Since the contacts are deionized only against the small voltage dropacross the conducting electronic switch 40, arcing quickly stops. Assoon as the electronic switch starts conducting, current stops flowingin interrupter 42. During this time, the circuit is again effectivelyalmost short circuited so that the current rises a small amount at timed, as indicated in Fig. 2. The amount of time switch 40 is conductive isshort, in the order of 1 ms., so that little current rise occurs.

After the first consecutive interrupter contacts 42 are fully open towithstand the surge voltage, control unit 54 again causes the tumofi ofelectronic switch 40 to bring the voltage back up to its peak point, asillustrated at time d in FIG. 3. The current is forced to flow throughresistor 46 and second consecutive interrupter contacts 46. During thetime period from d to e, the current is reduced to a point where it canbe accommodated by surge capacitor 52 with its surge suppressionresistor 50. This capability occurs at point e whereupon control unit 54causes opening of second consecutive interrupter contact 46 and theshort period of conduction of electronic switch 40. When the contacts 46are deionized, electronic switch 40 is again turned off by the controlunit 54 and the balance of the surge current in the system is directedinto surge capacitor 52 and resistor 50. The current has been brought tozero and the applied voltage across the buses is at the nominal value ofsource 18, as illustrated in FIGS. 2 and 3. The circuit breaker 12 canbe employed as a main switch for opening the bus, either at the sourceand/or load end thereof. Furthermore, it can be employed as a switch fora branch line on a transmission line. Thus, the circuit breaker I2 is aspecial purpose application of a generic switch.

This invention having been described in its preferred embodiment, it isclear that it is susceptible to numerous modifications and embodimentswithin the ability of those skilled in the art and without the exerciseof the inventive faculty. Accordingly, the scope of this invention isdefined by the scope of the following claims.

We claim:

1. A DC circuit interrupter comprising a DC electric line and transferswitch, contacts connected in said DC electric line to open said DCelectric line; i

an electronic switch capable of interrupting DC current initiallyflowing therethrough connected in parallel to said transfer switchcontacts;

a series combination of first consecutive interrupter contacts and firstresistor connected in parallel to said transfer switch contacts;

a series combination of second consecutive interrupter contacts and aresistor connected in parallel to said transfer switch contacts, acapacitor connected across said second consecutive interrupter contacts;and

control means connected to operate said transfer switch contacts, saidelectronic switch, said first consecutive interrupter contacts and saidsecond consecutive interrupter contacts for successive opening of saidtransfer tronic switch to be conductive during each of the successiveopenings and nonconductive intermediate the openings so that saidelectronic switch is conductive during opening of said contacts to stopcurrent flow through said contacts and permit them to deionize.

2. The interrupter of claim 1 wherein said first and second resistorsrespectively in series with said first and second consecutiveinterrupter contacts are nonlinear resistors.

3. The interrupter of claim 2 wherein a series combination of a surgecapacitor and a surge suppression resistor is connected in parallel tosaid transfer switch contacts to absorb current upon electronic switchopening.

4. The interrupter of claim 3 wherein said series connected surgecapacitor and surge suppression resistor are connected in parallelacross said second consecutive interrupter contacts.

5. The interrupter of claim 4 wherein fault sensing means is connectedto said DC line to detect a fault therein, said fault sensing meansbeing connected to said control means so that upon the sensing of afault in said DC line, said control means causes said interrupter toopen.

6. The interrupter of claim 5 wherein said electronic switch is acrossed field switch tube.

7. The interrupter of claim 5 wherein said electronic switch is a liquidmetal cathode switch tube.

8. The interrupter of claim 1 wherein fault sensing means is connectedto said DC line to detect a fault therein, said fault sensing meansbeing connected to said control means so that upon the sensing of afault in said DC line, said control means causes said interrupter toopen.

9. The interrupter of claim 1 wherein said electronic switch is acrossed field switch tube.

10. The interrupter of claim 1 wherein said electronic switch is aliquid metal cathode switch tube.

11. The method of interrupting a DC circuit having direct currentflowing through transfer switch contacts serially connected in thecircuit and having connected in parallel to the transfer switch contactsand in parallel to each other an electronic switch, a first consecutiveinterrupter having serially connected contacts and resistance, and asecond consecutive interrupter having serially connected contacts andresistance comprising the consecutive steps of:

opening the transfer switch contacts while the electronic switch isconductive to permit deionization and discontinuance of current flowthrough the transfer switch contacts; making the electronic switchnonconductive while said contacts of the first consecutive interrupterare closed to cause DC current flow through said the consecutiveinterrupter contacts and its respective series resistance to increasecircuit impedance and reduce current flow therein; opening the contactsof the first consecutive interrupter while the electronic switch isconductive to permit deionization and discontinuance of current flowthrough the contacts of the first consecutive interrupter; andterminating conduction through the electronic switch while the contactsof said second consecutive interrupter are closed to cause current fiowthrough the consecutive interrupter contacts and respective resistanceto further increase circuit impedance. 12. The process of claim 1including the further additional steps of:

' opening the contacts of the second consecutive interrupter while saidelectronic switch is conductive to permit deionization anddiscontinuance of conduction through the contacts of said secondconsecutive interrupter; and

causing discontinuance of conduction through the electronic switch andabsorbing the off-switching voltage pulse in a capacitor connected inparallel to said transfer switch contacts.

* k k i 253 3 UNITED S ATES PATENT OFFICE CERTIFICATE OE CORRECTIONPatent 3,641,358 Dated v February 8, 1972 l Kenneth T. Lian et al It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 6, line 64, cancel "1" and substitute ll. (claim 12, line 1)Signed and sealed this 8th dayofMay 1973.

(SEAL) Attest:

EDWARD I-LFLETgIHERJR. B T GOTTSCHALK Attesting Officer -v Commissionerof Patents

1. A DC circuit interrupter comprising a DC electric line and transferswitch, contacts connected in said DC electric line to open said DCelectric line; an electronic switch capable of interrupting DC currentinitially flowing therethrough connected in parallel to said transferswitch contacts; a series combination of first consecutive interruptercontacts and first resistor connected in parallel to said transferswitch contacts; a series combination of second consecutive interruptercontacts and a resistor connected in parallel to said transfer switchcontacts, a capacitor connected across said second consecutiveinterrupter contacts; and control means connected to operate saidtransfer switch contacts, said electronic switch, said first consecutiveinterrupter contacts and said second consecutive interrupter contactsfor successive opening of said transfer switch contacts, said firstconsecutive interrupter and said second consecutive interrupter and forcausing said electronic switch to be conductive during each of thesuccessive openings and nonconductive intermediate the openings so thatsaid electronic switch is conductive during opening of said contacts tostop current flow through said contacts and permit them to deionize. 2.The interrupter of claim 1 wherein said first and second resistorsrespectively in series with said first and second consecutiveinterrupter contacts are nonlinear resistors.
 3. The interrupter ofclaim 2 wherein a series combination of a surge capacitor and a surgesuppression resistor is connected in parallel to said transfer switchcontacts to absorb current Upon electronic switch opening.
 4. Theinterrupter of claim 3 wherein said series connected surge capacitor andsurge suppression resistor are connected in parallel across said secondconsecutive interrupter contacts.
 5. The interrupter of claim 4 whereinfault sensing means is connected to said DC line to detect a faulttherein, said fault sensing means being connected to said control meansso that upon the sensing of a fault in said DC line, said control meanscauses said interrupter to open.
 6. The interrupter of claim 5 whereinsaid electronic switch is a crossed field switch tube.
 7. Theinterrupter of claim 5 wherein said electronic switch is a liquid metalcathode switch tube.
 8. The interrupter of claim 1 wherein fault sensingmeans is connected to said DC line to detect a fault therein, said faultsensing means being connected to said control means so that upon thesensing of a fault in said DC line, said control means causes saidinterrupter to open.
 9. The interrupter of claim 1 wherein saidelectronic switch is a crossed field switch tube.
 10. The interrupter ofclaim 1 wherein said electronic switch is a liquid metal cathode switchtube.
 11. The method of interrupting a DC circuit having direct currentflowing through transfer switch contacts serially connected in thecircuit and having connected in parallel to the transfer switch contactsand in parallel to each other an electronic switch, a first consecutiveinterrupter having serially connected contacts and resistance, and asecond consecutive interrupter having serially connected contacts andresistance comprising the consecutive steps of: opening the transferswitch contacts while the electronic switch is conductive to permitdeionization and discontinuance of current flow through the transferswitch contacts; making the electronic switch nonconductive while saidcontacts of the first consecutive interrupter are closed to cause DCcurrent flow through said the consecutive interrupter contacts and itsrespective series resistance to increase circuit impedance and reducecurrent flow therein; opening the contacts of the first consecutiveinterrupter while the electronic switch is conductive to permitdeionization and discontinuance of current flow through the contacts ofthe first consecutive interrupter; and terminating conduction throughthe electronic switch while the contacts of said second consecutiveinterrupter are closed to cause current flow through the consecutiveinterrupter contacts and respective resistance to further increasecircuit impedance.
 12. The process of claim 1 including the furtheradditional steps of: opening the contacts of the second consecutiveinterrupter while said electronic switch is conductive to permitdeionization and discontinuance of conduction through the contacts ofsaid second consecutive interrupter; and causing discontinuance ofconduction through the electronic switch and absorbing the off-switchingvoltage pulse in a capacitor connected in parallel to said transferswitch contacts.